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The big picture

If you are brand new to Java, this chapter is here to give you a really quick introduction to what Java looks like, and how it works.

If you have worked with Java before, this chapter is mainly just to remind you of some things–but its still worth reading through just to dust off the cobwebs of your knowledge. Who knows, there might be something that is new, or presented in a new light.

By Sections

The way Java works

Get familiar with the terms:

  • Source code
  • Compiler
  • Bytecode
  • Virtual Machine

What you’ll do in Java

Note the role of:

  • javac command
  • java command
  • file
  • Party.class file

A very brief history of Java

In this class, we’ll be using Java 17 (even though there are higher versions of Java available.) The reason is that Java 17 is a “Long Term Support” version.

Also note that Java is much faster than it used to be, thanks mostly to improvements in the Java Virtual Machine.

Code structure in Java

Get familiar with the basic ideas here:

  • One class == One file
  • The top level is typically a file containing one class called Dog: ```java public class Dog {


Later in the course, we’ll see a few examples of something called inner classes: “classes inside other classes”, but there is still typically just one top level public class per file.

Like many rules of thumb, it turns out the real situation is a bit messier: see this Stack Overflow Discussion.

But, at least for now, its best to stick to one class per file.

Anatomy of a class

Not every class has to have a public static void main(String [] args) method, but you need a main() method to get a Java application started.

Contrast this with C++ where:

  • the int main() method stands alone outside any class
  • There can only be one main() method per application

In Java, by contrast:

  • Every main method has to live inside some class
  • There can be many classes that have a main method

The one that gets invoked is specified when you start up the JVM with,for example, the command java Dog

  • This looks for a class file Dog.class and starts executing the main method of that class
  • Where it looks for Dog.class is determined by something called the CLASSPATH, which we’ll get into later.

Writing a class with a main()

You definitely need to know how to write a simple Java Hello World! type program, and this section will teach you how.

What can you say in the main method?

This goes over some of the basics of Java syntax, which as you’ll notice, are pretty similar to C++ in many ways.

Looping and looping and…

Again, basic syntax details that are pretty similar to C++, but worth reviewing.

The main content here is again nothing too surprising, but the Q and A in this section is worth taking a little time with.

In particular, it is good to know about the way that in Java, unlike C/C++, you CANNOT just treat ints as if they were booleans. Good to know!

Will the following code print foo forever? Or is it a syntax error? Explain. (Assume it appears inside a main function)

int x = 1;
while (x) { 

Conditional branching

More basic syntax details that are pretty similar to C++, but worth reviewing.

Also covered, and something to notice: the difference between:

  • System.out.println
  • System.out.print

Coding a serious business application

This is just a version of “99 bottles of beer on the wall”; it’s worth reviewing to make sure the syntax makes sense to you.

Monday morning at Bob’s Java-enabled house

This is a pretty silly section, not gonna lie.


This is also kind of silly, but does introduce a few useful Java concepts, such as:

  • the syntax for hard coded arrays of String values
  • how to find the length of an array
  • how to select a random element of an array
  • working with String concatenation